1. Field of the Invention
The present invention relates to an image recording apparatus employing a cathode-ray tube (inclusive of an optical fiber tube) as an exposure means. More specifically, the invention relates to an image recording apparatus employing, as an exposure means, a cathode-ray tube in which a fluorescent surface is scanned by an electron beam.
2. Description of the Prior Art
A cathode-ray tube, for example, an optical fiber tube used in the image recording apparatus of this type has a flat fluorescent surface. Therefore, the distance between an electron gun for emitting an electron beam and the fluorescent surface is small at the central portion of the fluorescent surface and increases toward the end portions. As a matter of course, therefore, brightness is high at the central portion and decreases toward the end portions. The above-mentioned problem which stems essentially from the construction can be solved by a method in which the grid voltage is controlled to a predetermined value in synchronism with the scanning of electron beam, i.e., solved by a method of increasing the grid voltage at the time of scanning the end portions, or by a method according to which brightness during the scanning is measured by a photosensor, and a brightness correction signal is produced based upon the measured data to change the grid voltage, in order to make uniform the brightness in the central portion and the end portions.
On the other hand, irregular brightness also stems from irregularly applied fluorescent material. The problem of irregular brightness of this type is not satisfactorily solved by the above-mentioned two methods. This is because, irregularly applied fluorescent material produces irregular brightness. Therefore, of the above-mentioned two methods, the former method is not at all effective to cope with it. Even with the latter method, the limit is determined by the resolving power of photosensor. The irregular brightness further results from varying sweep speed of the electron beam. In either case, irregular brightness turns out to be irregular exposure for the photosensitive members. In the case of the optical fiber tube, irregular exposure is also caused by variance in the transmission quantity that is affected by the condition of each fiber of the optical fiber array (e.g. affected by the condition of end surface of the optical fiber).
With the current technique, it is difficult to remove the problem of irregular exposure as mentioned above. Therefore, when a hard copy is to be obtained by using the image recording apparatus of this kind, there will be no problem when a binary image is to be dealt with. A great problem, however, arises when a hard copy of continuous tone image is to be obtained. That is, irregular exposure in the direction of main scanning produces blurring of lines in the subscanning direction on the image that is recorded by moving the photosensitive member in the subscanning direction, and the image quality is deteriorated. For instance, a blur of white lines or black lines in the subscanning direction appears in the black-and-white continuous tone image.
FIG. 1 shows construction of a known apparatus for recording color image by using cathode-ray tubes of the scanning type. According to this construction, image data of corresponding colors are given to cathode-ray tubes 1', 2' and 3' of which the fluorescent surfaces are coated with a red fluorescent material, a green fluorescent material and a blue fluorescent material, respectively, and red light, green light and blue light produced from these cathode-ray tubes 1', 2' and 3' are superposed by a synthesizing prism 4' and are permitted to be incident upon a film (exposed portion) 7' via an image-forming lens 5' and a tilted reflector mirror 6'. Reference numeral 8' denotes a stage which is moved right and left by a step motor (not shown). The mirror 6' and the film 7' move on the stage 8' so as to be subjected to the subscanning. This recording apparatus, however, requires three cathode-ray tubes 1' to 3', resulting in increased size.
Another known conventional apparatus employs a single cathode-ray tube of which the surface to be scanned with electron beam is coated with fluorescent materials of three colors, i.e., red, green and blue, in the form of stripes. In this recording apparatus, exposure of each color is separately effected. Namely, the exposed portion is returned to the initial position three times repetitively, and the exposure is effected based upon the image data of each of the colors during each carry, to obtain a color image. This apparatus is free from the defects of the above-mentioned conventional apparatus. However, a complex mechanism is employed to return the exposed portion to the initial position, and to so carry the above portion again that images of each of the colors are superposed. Therefore, the apparatus becomes bulky. Despite the bulky apparatus, position for exposure to three colors lights deviates greatly, and a good color image is not obtained.
Further, in a recording cathode-ray tube used in a color recording apparatus, a fluorescent material applied onto the bombarding surface (back side of the light-emitting surface) is scanned with the electron beam, and the exposure is effected by utilizing the emitted light. For example, a white fluorescent material is applied onto the whole surface to which the electron beam will be bombarded, and a filter for blue color, a filter for green color and a filter for red color are stuck in the form of stripes onto the light-emitting surface (surface of the tube), to obtain blue light, green light and red light.
The white fluorescent material, however, is not capable of emitting the light in constant quantity with regard to the wavelength. A P4 fluorescent material, which is generally used as a white fluorescent material, emits the light in small amounts in a wavelength region of red color (600 nm and larger). Therefore, if it is attempted to obtain blue light, green light and red light by passing the light emitted by the white fluorescent material through filters in a customary manner, the intensity of red light becomes smaller than the lights of other colors.
It has also been proposed to apply three fluorescent materials for obtaining blue color, green color and red color in the form of stripes onto the bombarding surface of the recording cathode-ray tube onto which the electron beam will be bombarded. To apply three fluorescent materials in the form of stripes, however, involves considerable technical difficulty.